Store and read parameter library as sysdata

[joshwlambert]

This PR addresses #198 by adding sysdata.rda to the package and reading from sysdata in epidist_db(). To optimise the performance of reading the epidemiological parameters into the R environment, the database is not read and formatted as <epidist> objects each time.

The code that creates the sysdata is added to data-raw/parameters.R.

A test is added for .read_epidist_db() as this code is no longer run when epidist_db() is called (unless the sysdata cannot be located, a situation which cannot be mocked in testing).

The dev-utils is updated to remind updating the sysdata at every release. The .Rbuildignore is updated to include data-raw.

[joshwlambert]

@TimTaylor thanks for reviewing! Please approve the PR if you're happy with the changes.